In MR Imaging systems radio frequency coils are used to transmit the radio frequency signals which nutate nucluei that have been aligned by the static magnetic field. After the Rf signal is removed, the nutated nucluei subsequently return to their aligned positions. During this return the free induction decay (FID) signals are generated. These signals are detected by Rf coils and provide the image data used to generate the display images.
It is understandably extremely important that the RF coils be efficient in the transmission of Rf power signals with a maximum signal to noise ratio. The prior art basically uses shaped wire coils. The prior art coils are coupled to the Rf modulator at one point in the coil. It would be advantageous both from a power efficiency stand-point and from a signal-to-noise ratio stand point to improve the over-all efficiency of Rf probes used in MRI system. Improvements in power efficiency and in th signal-to-noise ratio effect the readability and resolution of the images obtained from the MRI system.
Accordingly it is an object of the present invention to provide improved Rf coils for MRI systems.
In accordance with the present invention an improved Rf probe for MRI systems is provided, said probe comprises:
a loop for generating a magnetic field responsive to the passage of a varying current at Larmor frequencies, PA0 means for coupling electro-motive forces (EMF) to said loop to cause the passage of current through said loop, and PA0 said coupling means comprising at least two feed points in said loop whereby the EMF causes the current to flow from the instantaneous positive voltage at the first feed point to the instantaneous negative voltage at the second feed point and from the instantaneous positive voltage at the second feed point to the instantaneous negative voltage at said feed first point.
A feature of the invention provides for decreasing the electrical length of the Rf coil while at least maintaining the magnetic strength of the magnetic field generated by the Rf coil. Another feature of the invention provides for decreasing the Re of the probes where Re is the loss due to the coupling to the lossy dielectric material in the specimen being imaged.